The study looked at how HIV-1 mutates and the impact on its ability to replicate. They found that mutations occur at a rate of 1.2 × 10−5 per site per day, with certain types of mutations more common than others. Mutations that change the genetic code have a bigger impact on virus replication than those that don't. Most harmful mutations reduce virus replication by more than 10 percent, while harmless mutations have less than a 1 percent impact. The cost of mutations is consistent across different patients, suggesting that the negative effects on virus fitness are universal and play a big role in the diversity of HIV-1.